Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore, other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersions used in coating processes are usually in a concentrated form and contain a significant quantity of nonionic surfactant, e.g. 6-8 weight percent, as taught in Marks et al., U.S. Pat. No. 3,037,953. Similar dispersions and coating processes are also used for making coatings of melt-processible fluoropolymers.
For some specialized fluoropolymer dispersion applications, common nonionic surfactants such as alkyl phenol ethoxylates or aliphatic alcohols ethoxylates for stabilization are unsuitable. One such application is the manufacture of filled fluoropolymer bearings where anionic surfactants have been used for stabilization of dispersions during handling and transportation. To prepare the fluoropolymer for bearing manufacture, an aluminum salt, usually the nitrate or sulfate, is added which reacts with an anionic surfactant such as sodium lauryl sulfate to form insoluble aluminum lauryl sulfate. The PTFE is thereby coagulated due to the loss of surfactant, forming a thick mixture referred to as a “mush”. The end user adds a suitable filler such as lead powder, molybdenum disulfide or other suitable filler for the particular bearing application either prior to or after formation of the mush. A small amount of non-ionic surfactant or a water immiscible organic, usually toluene, is typically added to help wet the filler and control the viscosity of the “mush”. The mush is calendared onto metal strips and sintered to form a coated substrate which is formed into a bearing. Filled fluoropolymer bearing, due to lubricating properties of the PTFE and/or fillers, can be used in dry un-lubricated form, i.e., without lubricating oil or grease. Typical dispersions stabilized with nonionic surfactants are much more difficult to coagulate and do not form acceptable mushes for use in filled fluoropolymer bearing manufacture.
Fluorosurfactants are typically used as an ingredient in the dispersion polymerization of fluoropolymers, the fluorosurfactants functioning as a non-telogenic dispersing agent. For example, an early description of this use of fluorosurfactants is found in U.S. Pat. No. 2,559,752 to Berry. However because of environmental concerns and because fluorosurfactants are expensive, processes have been developed for reducing and recovering fluorosurfactant from aqueous fluoropolymer dispersions.
One common method is to remove fluorosurfactant by adsorption onto an ion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls) and U.S. Pat. No. 6,833,403 (Blade) et al.) For effective removal, such dispersions are stabilized with a nonionic surfactant, such as alkyl phenol ethoxylates or aliphatic alcohol ethoxylates as disclosed in U.S. Pat. No. 3,037,953 to Marks et al.; U.S. Pat. No. 6,153,688 to Miura et al.; and U.S. Pat. No. 6,956,078 to Cavanaugh et al. Dispersions stabilized with nonionic surfactant are used since removal of the fluorosurfactant without nonionic surfactant being present generally results in coagulation of the dispersion.
If it is attempted to use an anion exchange process for the fluoropolymer dispersions discussed above that are stabilized with an anionic hydrocarbon surfactant such as sodium lauryl sulfate instead of nonionic surfactant, the anionic hydrocarbon surfactant will be removed from the dispersion together with the fluorosurfactant causing coagulation of the dispersion.